Characterisation of Tn1721, a new transposon containing tetracycline resistance genes capable of amplification.

R plasmid pRSD1 contains tetracycline resistance (tet) genes in a 3.55 Mdal-region capable of amplification by forming tandem repeats (Mattes, Burkardt and Schmitt, Molec. gen. Genet., 1979). The repetitious tet element is itself part of a 7.2 Mdal-transposon, named Tn1721, as demonstrated by the following criteria; (i) Tn1721 has been translocated to phage lambda. The resulting hybrid phage lambda tet contains the 7.2 Mdal-insertion to the right of the attachment site, but not continguous with it indicating translocation of the element by non-homologous recombination. In addition, lambda tet has sustained a 3.4 Mdal-deletion adjacent to the insertion. (ii) Further transposition of Tn1721 to the 21.5 Mdal-plasmid R388 resulted in R388::Tn1721 derivatives, two of which were characterised. They contain Tn1721 inserted into different sites but in the same orientation as shown by restriction and heteroduplex analyses. These translocation of Tn1721 were not accompanied by deletions of DNA. (iii) The insertion plasmid pRSD102(R388::Tn1721) has conserved the capacity of the original plasmid pRSD1 to amplify the 3.55 Mdal-tet region. It has been concluded that Tn1721 constitutes a novel transposon encompassing a tet region capable of selective amplification. The model proposed for Tn1721 contains three short repeats. Two direct repeats, flanking the 3.55 Mdal tet region, provide sequence homology for amplification. The third repeat (located distally to tet) is inverted and provides the basis for transposition of the 7.2 Mdal-element.